 /*******************************************************************************
  * Copyright (c) 2004, 2006 IBM Corporation and others.
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License v1.0
  * which accompanies this distribution, and is available at
  * http://www.eclipse.org/legal/epl-v10.html
  *
  * Contributors:
  * IBM Corporation - initial API and implementation
  *******************************************************************************/
 package org.eclipse.ui.internal.themes;

 import java.util.Collections ;
 import java.util.HashSet ;
 import java.util.Map ;
 import java.util.Set ;

 /**
  * @since 3.0
  */
 public class CascadingMap {

     private Map base, override;

     /**
      * @param base the base (default) map
      * @param override the override map
      */
     public CascadingMap(Map base, Map override) {
         this.base = base;
         this.override = override;
     }

     /**
      * Return the union of the parent and child key sets.
      *
      * @return the union. This set is read only.
      */
     public Set keySet() {
         Set keySet = new HashSet (base.keySet());
         keySet.addAll(override.keySet());
         return Collections.unmodifiableSet(keySet);
     }

     /**
      * Get the value. Preference will be given to entries in the override map.
      *
      * @param key the key
      * @return the value
      */
     public Object get(Object key) {
         if (override.containsKey(key)) {
             return override.get(key);
         }
         return base.get(key);
     }
 }

